1. Field of the Invention
The present invention relates to a branch pipe forming tool and a method of forming a branch pipe on a metal tube with the tool, and more specifically, it relates to a branch pipe forming tool which can efficiently form a branch pipe on a metal tube with high shape accuracy and a method of forming a branch pipe on a metal tube with the tool.
2. Description of the Background Art
In general, metal tubes are employed as protective tubes or conduits for enclosing wiring cables which are buried under the ground. Among such metal tubes, a metal tube 20 having a branch pipe 20b shown in FIG. 15 is generally employed. The branch pipe 20b can be formed on the metal tube 20 by any of the following four methods, which are well known in the art:
The first method is adapted to form metal parts 20A and 20B shown in FIG. 16, which are half the metal tube 20 having the branch pipe 20b shown in FIG. 15 respectively, by press working, and thereafter connecting these metal parts 20A and 20B with each other by welding, thereby forming a metal tube having a branch pipe.
The second method is called "plug passing". As shown in FIG. 17, a plug 110 having a prescribed shape is drawn up from the interior to the exterior of a metal tube 20 along the direction of arrow A while its periphery is heated by high frequency induction heating, thereby outwardly raising up the tube wall and forming a branch pipe 20b.
The third method is adapted to form a branch pipe with a swinging tool 120. The swinging tool 120 of a prescribed shape is driven with a feed f and rotation .beta. as shown in FIG. 18, thereby gradually raising up the tube wall of a metal tube 20 and forming a branch pipe 20b.
The fourth method utilizes rubber. As shown in FIG. 19, confining tools 130 and 150 are arranged around a metal tube 20 while rubber 140 is injected into the interior of the metal tube 20 and pressed by plungers 160 from both sides of the metal tube 20, thereby outwardly raising up the tube wall of the metal tube 20 along the shape of the confining tool 130 and forming a branch pipe 20b.
However, the aforementioned four methods of forming branch pipes on metal tubes have the following problems respectively:
The first method requires the steps of pressing the metal parts 20A and 20B and connecting these metal parts 20A and 20B with each other. In particular, the pressing step requires molds which are responsive or adapted to the shapes of the metal tube and the branch pipe, which disadvantageously limits the type of the metal tube that can be provided with the branch pipe.
The second method requires the steps of previously forming a prepared hole in the metal tube 20 and setting the plug 110 in this prepared hole, and these steps must be carried out independently of each other. Further, it is necessary to heat the periphery of the plug by high frequency induction heating.
The third method requires the steps of forming a prepared hole in the metal tube and setting the tool in the prepared hole, similarly to the second method.
Finally, the fourth method requires the steps of previously forming a prepared hole in the metal tube, setting the confining tools on the metal tube, injecting the rubber into the metal tube, pressing the injected rubber with the plunger, and removing the rubber and the confining tools. Similarly to the first method, further, this method requires the confining tools which are responsive or adapted to the shapes of the metal tube and the branch pipe, and hence the type of the metal tube that can be provided with the branch pipe is disadvantageously limited.